Hokkaido
Hokkaido, the northernmost island of the four main islands, was formed
by which the Kuril Arc collided with the Northeast Japan Arc at the
central part of Hokkaido after the Middle Miocene. Therefore, Hokkaido
is divided into three parts: the eastern region in the Northeast Japan
Arc, the central region in the arc-arc collision zone, and the western
region in the Kuril Arc. The volcanic front runs nearly east-west from
the eastern region through the central region and turns to the south in
the western region. This front line is parallel to the Kuril Trench and
the Japan Trench. It is thought that there was a plate boundary in the
central region and it shifted to the margin of the Sea of Japan in the
end of the Tertiary or the Early Quaternary.
The landforms of Hokkaido are characterized by the smaller area of
mountains than other main islands. The mountains in Hokkaido generally
have gentle slopes in the northern part and steep slopes in the
southern part.
For the basement rocks (geotectonic units), see also “Outline
of landforms and geology of Japan”.
Eastern region
In the eastern region, northeastward trending uplift areas with
volcanoes are arranged in echelon toward the Kuril Islands, including
the Akan volcanoes, the Shiretoko Peninsula, the Kunasir Island, and the
Etorofu Island. The en echelon arrangement of the uplift areas is
attributed to the oblique subduction of the Pacific Plate along the
Kuril Trench. The Konsen Plateau, the Kushiro Plain, and the Tokachi
Plain spread in the Kuril outer arc on the south of the volcanic area.
The volcanoes provided massive volcanic products and clastic material
produced by erosion of the volcanoes to the Konsen Plateau and the
Kushiro Plain to form plateaus and hills. This sediment covers
Cretaceous and Paleogene rocks. The Shiranuka Hills situated between the
Kushiro Plain and the Tokachi Plain are a low relief uplift area.
Mountains in the central part of the Shiranuka Hills comprise Cretaceous
and Paleogene rocks, and they are surrounded by hills consisting of
Neogene and Quaternary formations. Pliocene-Quaternary formations are
distributed in the Tokachi Plain.
The Kitami Mountains are placed in the Kuril inner arc to the east
of the Teshio Mountains. This mountain range is the
stablest area in Japan, in which active faulting and earthquakes rarely
occur. The Kitami Mountains
consist mainly of Cretaceous-Paleogene basement rocks and volcanic rocks
that erupted in the Miocene or later and covered the basement. The
volcanic rocks formed lava plateaus. The pre-Neogene rocks in the Hidaka
Supergroup consist of turbidite sandstone and mudstone, and mélange
including chert, limestone and greenstone.
The Kussharo volcano in the eastern region is accompanied with the
largest caldera (26 × 20 km) in Japan.
Central region
The Teshio Mountains and the Yubari Mountains are located north-south in the central region. The Teshio Mountains consist of Cretaceous-Tertiary folded formations affecting the morphology. The Yubari Mountains have Jurassic-Cretaceous formations (Sorachi Group consisting of greenstone [including basaltic pillow lava, hyaloclastite, and diabase], chert, micrite limestone, and sandstone with felsic tuff) and serpentinite in and around the main ridge. Cretaceous forearc sediments (Yezo Supergroup) are distributed around the formations and Paleogene formations with interbedded coal seams are found on the west of the Cretaceous sediments. These mountains were upheaved by the collision of the two island arcs.
The Hidaka Mountains in the western margin of the Kuril outer arc are
situated to the southeast of the Yubari Mountains. This mountain range
was also uplifted in association with the colliding Kuril Arc. A
depression zone is placed between the Teshio-Yubari Mountains and
the Kitami-Yubari Mountains. In the Hidaka Mountains, the Hidaka metamorphic
rocks, the Cretaceous-Paleogene system, and the Cretaceous system are
distributed in the central part, eastern part, western part,
respectively (Figure). The crust of the island arc and the upper mantle
were thrusted up on the metamorphosed oceanic crust along the Hidaka
thrust fault. The Hidaka metamorphic zone, therefore, is regarded as
the exposed cross section of the island arc crust. High pressure type
metamorphic rock zone (Kamuikotan Belt) is found on the west of the
Hidaka metamorphic zone.
The Daisetsu and Tokachi volcanic area (Ishikari Mountains) between the
Hidaka Mountains and the Kitami Mountains are the highest elevation area in
Hokkaido (the highest peak is 2290 m high).
Western region
The Ishikari Plain is an alluvial lowland to the west of the Yubari
Mountains. The Oshima Peninsula is located on the west of the Ishikari
Plain, which is part of the Northeast Japan Arc.
Mountains, volcanoes, and plains in the peninsula are distributed
complicatedly. This irregular arrangement is a different feature from
the central and eastern regions with the zonal distribution of
landforms. A Jurassic accretionary complex intruded by Cretaceous
granite is found in this region, which is uncoformably covered with
Neogene-Quaternary volcanic rocks and formations.
Some volcanoes such as the Shikotsu volcano and the Toya volcano in
this region have large calderas and pyroclastic plateau.
References
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